1. Field of the Invention
The present invention is directed generally to safely delaying nuclear power plant protective functions and more specifically to the variable delay of steam generator low water level protective functions, i.e., reactor trip and auxiliary feedwater system actuation.
2. Description of the Prior Art
In present nuclear power plant protection systems, a low water level, in any steam generator, will trip the reactor and activate the auxiliary feedwater system. FIG. 1 illustrates one example of auxiliary feedwater system actuation and reactor trip logic in a plant with four steam generators. Low water level indicators associated with each steam generator drive logical OR gate 14 and logical AND gate 15. A low water level indication in one steam generator will cause a signal to be generated which is available at an output of OR gate 14 to thereby initiate a reactor trip and actuate the motor-driven auxiliary feedwater pumps. Likewise, a low water level indication in two or more steam generators will cause a signal to be generated which is available at an output of AND gate 15 to thereby actuate the turbine-driven auxiliary feedwater pump.
These actions are intended to protect the nuclear core and to maintain an adequate heat sink for decay heat removal. The most critical need for such protective action would occur while the plant is operating at full power following a total loss of feedwater to all steam generators or a major feedwater line rupture. Therefore, the steam generator low water level protection system logic and setpoints are determined according to the requirements of these limiting postulated conditions. The same protective functions also occur, however, under less limiting conditions such as the termination of feedwater to only one steam generator during plant startup operations.
A survey of nuclear power plant operating experience indicates that a large number of all unplanned reactor trips are attributable to low steam generator water level trip signals. The removal of unnecessary conservatisms that are inherent in the accident analysis assumptions and models and that are applied to determine the steam generator low water level reactor trip setpoint would permit the low water level trip setpoint to be selectively lowered during plant operation under less-than-limiting conditions and thereby reduce the likelihood that the steam generator water level would fall below the trip setpoint and unnecessarily trip the reactor. Unfortunately, lowering the steam generator low water level trip setpoint is constrained by the physical design of the steam generator. Accordingly, the need exits for a system which is capable of eliminating unnecessary reactor trips.